Note that this (using STET() in condition part of IF() rule) only works as of version 5.1.

If someone enters a bigger value, it will still be in the database; but when value is fetched in normal data retrieval, it will return 750 as maximum value. Keep in mind that some ways of data retrieval (say, Cube export) will not use rule-calculated values by default, in those cases, you'd still get the bigger value.

If you wanted to be sure that the actual value in the cell never exceeds 750, you'd have to use Supervision-Server to validate all inputs and make sure inputs larger than <maximum> are refused.